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PLL Synthesizers


DN006: Understanding Phase Noise from Digital Components in PLL Frequency SynthesizersThe article explains how the phase noise from the digital components in a PLL synthesizer (dividers, phase detector and charge pump) can be understood in terms of timing jitter in these devices. This simple physical model is shown to predict the observed variation of phase noise with PLL parameters and provides further insight into sources of significant phase noise. Download pdf file (61kB)
SimPLL TutorialThis is not an article but an interactive phased lock loop tutorial. Whilst designed initially to introduce users to the SimPLL program, it demonstrates many significant issues that occur in PLL design (phase noise tradeoffs, non-linear effects in lock times, reference spurs and more). Only the FREE demo version of SimPLL is needed to run the tutorial. Details
Phase Noise ReferenceIntroducing phase noise, sources and definitions, along with how to calculate the effects of phase noise on system performance. Includes adjacent channel interference, adjacent channel rejection, reciprocal mixing, residual FM and more. Details
DN005: Charge Pump Saturation Effects in PLL Frequency SynthesizersThere is a push to reduce IC voltages everywhere, providing a temptation to reduce the voltage available to the charge pump phase detector in modern PLL IC’s, which in turn limits the voltage swing to the VCO. Even if the phase detector output swing comfortably covers the required VCO tuning range, charge pump limiting can have a significant impact on the lock time, as detailed in this article. Download pdf file (40kB)
DN003: Measuring the Loop Bandwidth of a PLLThis describes a technique for measuring the loop response (amplitude and phase) of a PLL. Actually measure the loop bandwidth and the phase margin! Download pdf file (80kB)
DN002: Identifying Phase Noise Sources in a PLLThis report details a simple technique for restricting PLL bandwidth to enable the phase noise of the VCO alone to be measured, whilst still locked in the PLL. Download pdf file (94kB)

RF Circuits
DN004: Group Delay – Explanations and ApplicationsA discussion of several of the hard to understand aspects of group delay. Why must filters have group delay, how is it related to signal delay and the relationship to excess loss in passive filters are some of the areas discussed. Download pdf file (75kB)





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  1. PLL

    The Swallow Counter is one of the three building blocks (swallow counter, main counter, and dual-modulus prescaler) that constitute the programmable divider commonly used in modern frequency synthesizers.
    The swallow counter is used to control the dual-modulus prescaler which is set to either N or (N+1). At the initial reset state, the prescaler is set to a divide ratio of (N+1), but the swallow counter will change this divide ratio to N when it finishes counting S number of cycles.

    The Swallow Counter gets its name from the idea that it “swallows” 1 from (N+1) of the dual-modulus prescaler.






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